The present invention relates to an evacuated heat insulation unit (hereinafter referred to as insulation unit) for use in, for example, refrigerators as a heat insulation panel.
The insulation unit for use in refrigerators has a synthetic resin bag charged with a heat insulation core material, such as a foamed and then crashed fine perlite powder, in a vacuum state. The evacuated insulation unit has a high heat insulating property or a low thermal conductivity of about 0.01 Kcal/m.h oc or less since it includes a heat insulation vacuum layer. However, the evacuated insulation unit is degraded in its heat insulating property with lapse of time since a small amount of air permeates through the plastic bag into the evacuated insulation unit.
When the opposite films of the insulation bag are made of a metallic foil, the evacuated heat insulation bag is excellent in vacuum degree retention since a metallic foil allows only a trace amount of gas to permeate it and hence the insulation unit has a prolonged life. However, the metallic foils of the insulation unit constitute at the sealed peripheries a heat bridge between the outer face and the inner face of the insulation unit, and hence heat conductivity at the peripheries of the insulation unit is rather large, resulting in deterioration in insulation property of the whole insulation unit.
To avoid this, an attempt has been made to form a bag for the insulation unit with plastic films, made of a polyethylene, polyester, etc, and vapor deposited with a metal such as aluminum. However, this metallized film bag allows small gas permeation and hence the life of the insulation unit using this bag is not sufficient.
A further attempt has been made to prolong the life of the insulation unit by the use of a bag using a metallic foil laminated plastic film. Although this metallic foil laminated bag ensures a fairly long life of the insulation unit, there arises the same heat bridge problem as in the metal foil bag previously mentioned. It was noted that in a 500.times.500.times.20 mm insulation unit using a laminated film bag laminated with a 20 .mu.m aluminum foil, heat which passes through the peripheries thereof was about five to ten times in quantity as large as heat which passes through a planar portion surrounded by the peripheries. There is a tendency that the smaller the insulation unit, the larger this phenomenon.